Resumen: Mannheimia haemolytica–associated abomasitis has been clinically described as a cause of sudden death in lambs, but it is poorly characterized. We describe the pathological features of a severe fibrinonecrotizing abomasitis in 3 lambs that died suddenly. All 3 abomasums had a thickened submucosa due to edema and necrotic areas delimited by bands of degenerate neutrophils with slender nuclei (oat cells) and angiocentric distributions. The overlying mucosa was congested. Myriads of gram-negative coccobacilli were observed within the oat cell bands. M. haemolytica was isolated from the abomasum in all 3 animals and was serotyped as A2 in one of them. Pericarditis and pleuritis were observed in 2 of the lambs. Clostridium spp. were isolated in 1 lamb and detected by immunohistochemistry in the 3 animals, suggesting clostridial co-infection. M. haemolytica should be considered among the differential diagnoses of necrotizing abomasitis in lambs. Idioma: Inglés DOI: 10.1177/03009858241235393 Año: 2024 Publicado en: Veterinary Pathology 61, 4 (2024), 604-608 ISSN: 0300-9858 Tipo y forma: Article (Published version) Área (Departamento): Área Sanidad Animal (Dpto. Patología Animal) Área (Departamento): Área Anatom.Anatom.Patológ.Com (Dpto. Anatom.,Embri.Genét.Ani.)